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Tying wires for reinforcement adjacent to and above Class F4 and F5 finishes should be stainless steel wires. Why?

If plain steel tying wires are used for reinforcement adjacent to Class F4 and F5 finishes, it poses the problem of rust staining which may impair the appearance of exposed concrete surfaces. The rate of corrosion of plain steel tying wires is similar to normal steel reinforcement. However, for tying wires with very small diameter, upon long exposure it stands a high chance of rusting completely and these rust will stain the formwork and significantly affect the concrete finish. Therefore, stainless steel tying wires are specified for locations in the vicinity of high quality of finishes to avoid rust staining by corroded typing wires.

Note: Tying wires are wires used for fixing and connecting steel reinforcement bars.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What is the effect of rusting on steel reinforcement?

The corrosion of steel reinforcement inside a concrete structure is undesirable in the following ways:

(i) The presence of rust impairs the bond strength of deformed reinforcement because corrosion occurs at the raised ribs and fills the gap between ribs, thus evening out the original deformed shape. In essence, the bond between concrete and deformed bars originates from the mechanical lock between the raised ribs and concrete. The reduction of mechanical locks by corrosion results in the decline in bond strength with concrete.

(ii) The presence of corrosion reduces the effective cross sectional area of the steel reinforcement. Hence, the available tensile capacity of steel reinforcement is reduced by a considerable reduction in the cross sectional area.

(iii) The corrosion products occupy about 3 times the original volume of steel from which it is formed. Such drastic increase in volume generates significant bursting forces in the vicinity of steel reinforcement. Consequently, cracks are formed along the steel reinforcement when the tensile strength of concrete is exceeded.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

For column reinforcements, why is helical reinforcement sometimes designed instead of normal links?

The use of links for column design in Britain is very popular. However, in U.S.A. engineers tend to use helical reinforcement instead of normal links because helical reinforcement has the potential advantage of protecting columns/piles against seismic loads. Moreover, when the columns reach the failure state, the concrete outside hoops cracks and falls off firstly, followed by the eventual failure of the whole columns. The peeling off of concrete outside helical reinforcement provides a warning signal before the sudden failure of columns as suggested by G. P. Manning (1924). In addition, it can take up a higher working load than normal link reinforcement.

For instance, helical reinforcement is adopted in the design of marine piles in Government piers.

Note: Helical reinforcement refers to shear reinforcement which is spiral in shapes.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Why does the presence of tension reinforcement lead to increasing deflection in concrete structures?

In BS8110 a modification factor is applied to span/depth ratio to take into account the effect of tension reinforcement. In fact, deflection of concrete structure is affected by the stress and the amount of tension reinforcement.

To illustrate their relationship, let’s consider the following equation relating to beam curvature:

Curvature = 1/r = e/(d-x)
where r = radius of curvature
e = tensile strain in tension reinforcement
d = effective depth
x = depth of neutral axis

Provided that the tensile strain in tension reinforcement remains constant, the curvature of concrete structure increases with the depth of neutral axis. It is observed that the depth of neutral axis rises with tension steel ratio. Therefore, the curvature of concrete section is directly proportional to the tension steel ratio. In addition, the larger value of the depth of neutral axis enhances increased area of concrete compression so that the effect of creep on deflection appears to become significant.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What is the purpose of setting minimum amount of longitudinal steel areas for columns?

In some design codes it specifies that the area of longitudinal steel reinforcement should be not less than a certain percentage of the sectional area of column. Firstly, the limitation of steel ratio for columns helps to guard against potential failure in tension. Tension may be induced in columns during the design life of the concrete structures. For instance, tension is induced in columns in case there is uneven settlement of the building foundation, or upper floors above the column are totally unloaded while the floors below the column are severely loaded. Secondly, owing to the effect of creep and shrinkage, there will be a redistribution of loads between concrete and steel reinforcement. Consequently, the steel reinforcement may yield easily in case a lower limit of steel area is not established.

In addition, test results showed that columns with too low a steel ratio
would render the equation below inapplicable which is used for the design of columns:

N=0.67fcuAc+fyAs

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.